This invention relates to a process for efficiently cast molding a transparent polymeric product which is free from defects such as strain and has improved properties, for example, of transparency, weather resistance, impact resistance and heat resistance.
It is well known that transparent polymeric materials are substituting for some applications of inorganic glass by virtue of their advantageous properties such as light weight, improved fabricability (processability) and impact resistance. Their importance as materials for safety glass has also been recognized in recent years.
A transparent polymeric material that is currently most commonly used is polymethylmethacrylate, which is less expensive and has improved transparency, appearance, impact resistance, and weather resistance. For these advantages, the polymethylmethacrylate is used in a wide variety of applications such as interior decoration, displays, building materials like ceilings or doors, window glass of passenger vehicles, and aquaria. But in other respects, for example, surface hardness, heat resistance and chemical resistance, the polymethylmethacrylate is not highly desirable. The properties mentioned above can be improved by incorporating into the polymethylmethacrylate those polyfunctional monomers which are capable of providing polymers having high heat resistance or surface hardness, but then in turn, the thus treated polymethylmethacrylate becomes less thermoplastic and it is difficult to mold and process a copolymer from it. This difficulty in molding requires cast polymerization, but to produce a product free from such drawbacks as strain or bubbles, this process takes a long polymerization time. The production, for example, of polymeric product 1 cm thick in a plate glass form requires at least 24 hours. A thicker product is cast polymerized only after a period ranging from some 50-60 hours up to about 500-600 hours. This is without doubt too uneconomical for the commercial production of a desired cast molded article.
It is therefore plain to see that the development of a cast polymerization process for producing efficiently and rapidly those polymethylmethacrylates, representative of polymers of lower alkyl acrylates or methacrylates, which have improved heat resistance, chemical resistance and surface hardness will be greatly advantageous and contribute noticeably to commercial acceptance of transparent plastics materials in the art concerned. As such technique, this invention provides a process for cast polymerizing quickly as well as efficiently polymethylmethacrylate having improved properties.